1. Field of the Invention
The present invention relates to a gas sensor for detecting the concentration of a gas such as oxygen, nitrogen oxide, sulfur oxide, carbon monoxide, or carbon dioxide contained in exhaust gas discharged from an internal combustion engine, a combustion furnace, or the like.
2. Description of Background Art
A gas sensor has been used to detect the concentration of a gas such as oxygen, nitrogen oxide, sulfur oxide, carbon monoxide, or carbon dioxide contained in exhaust gas discharged from an internal combustion engine such as an automotive engine, a combustion furnace, an incinerator, or the like. For example, the amount of air (oxidizing agent) or fuel to be supplied is controlled by measuring the concentration of a specific gas contained in the exhaust gas of the internal combustion engine so that the combustion state in the internal combustion engine is suitably controlled based on the detected signal. This prevents emission of incompletely combusted gas, allows exhaust gas to be purified, and reduces fuel consumption. It is preferable that such a gas sensor have a high degree of selective reactivity with the gas to be measured, be chemically stable against high-temperature exhaust gas, allow easy manufacture, and be reduced in size. Therefore, a gas sensor utilizing a solid sensor element, such as an oxygen sensor utilizing a solid electrolyte having oxygen ionic conductivity or a carbon monoxide sensor utilizing the semiconductor properties of tin oxide, has been widely used (see Japanese Patent Application Laid-open No. 9-318594, for example).
Such a gas sensor is generally operated in a predetermined high temperature region. Therefore, a gas sensor having a structure in which a heater is provided therein is widely used. However, there may be a case where separation occurs inside the gas sensor during use.